served, is smooth and vertical. Each superior
maxillary bone contains the sockets of five teeth, occupying an
antero-posterior extent of three inches seven lines, (Pl. XXII and
XXIII. fig. 3). The posterior alveolus is situated just behind the
transverse line, extending across the anterior boundary of the orbits;
the remaining sockets of the molar series extend forwards three inches
in front of the orbits. In the Megatherium, the roots of the five
superior molars are all situated behind the anterior boundary of the
orbit: in the Orycteropus, on the contrary, the grinders are all
placed in advance of the orbit; so that the Scelidothere resembles that
species more than the Megathere in the relative location of the teeth.
The palatal interspace between the roots of the last molar tooth of
each series is eleven lines; the palate gradually though slightly
widens, as it advances forwards: the posterior margin of the palate is
terminated by an acute-angled notch. In the breadth of the bony palate
the Scelidothere is intermediate between the Megathere and Orycterope.

The anterior of the upper molars is represented at
fig. 3, 4, and 5, Pl. XXI., and at 1, fig. 3, Pl. XXIII.; it
corresponds
closely in form and size with the opposite molar below; the base of
the triangle given by its transverse section is turned inwards and
obliquely forwards.

The second molar of the upper jaw, also presents in
transverse section a triangular form, with the angles rounded off; but
the inner side of the tooth is traversed by a longitudinal groove. The
largest diameter of the transverse section, which is placed obliquely
as regards the axis of the skull, measures ten lines and a half; the
opposite diameter of the tooth is six lines.

The third and fourth molars present the same form
and size, and relative position as the second.

The fifth molar is the smallest of the series; its
transverse section gives an inequilateral triangle, with the corners
rounded off; the broadest side is turned outwards, and is slightly
concave; the antero-posterior diameter of this tooth is seven lines;
the transverse four lines. The length of the teeth in the upper jaw is
about two inches and a half.

It is almost superfluous to observe that the teeth
of the Scelidothere, as in other Bruta, are without fangs,
and
have their inserted base excavated by large conical cavities, for the
lodgment of a persistent pulp. The tooth is composed of a small central
body of coarse ivory or 'dentine,' traversed by medullary canals, which
at the periphery of the coarse dentine anastomose by loops, from the
convexity of which the calcigerous tubes are given off which form the
fine dentine: the layer of this substance, which immediately surrounds
the coarse dentine, is about one line and a half in thickness, and the
whole is invested with a very thin coating of cement. The teeth of the
Scelidothere thus

present a more resisting structure than do those of
the Mylodon; having a larger proportion of the dense ivory composed of
the minute calcigerous tubes, and a much smaller proportion of the
softer external cæmentum; in this respect the Scelidothere recedes
farther from Megathere, and approaches nearer the Armadillos than does
the Mylodon.

The lower jaw resembles, in the general form of the
posterior moiety which is here preserved, that of the Sloth and Mylodon
more than that of any other Edentate species. Its deep posterior angle
is produced backwards, and a broad coronoid process rises and nearly
fills the zygomatic space; the condyle is flat, as the glenoid surface
has already indicated; its transverse diameter is an inch and eight
lines; its antero-posterior diameter seven lines: it is principally
extended inwards beyond the vertical line of the ascending ramus. The
lower contour of the jaw describes an undulating line; which,
commencing from the posterior angle, is at first gently convex, then
slightly concave, then again convex, below the alveoli of the teeth,
where it is rounded and expanded, as in the Orycterope. The fractured
condition of the right ramus of this part fortunately exposed the roots
of the four grinding teeth, which constitute the dental series on each
side of the lower jaw. The length of the jaw occupied by these four
alveoli is three inches ten lines, which exceeds a little that of the
opposed five grinders above; the ramus of the jaw gradually diminishes
in all its dimensions anterior to the molar teeth; the dental canal
passes in a gentle curve below, and on the inner side of the alveoli,
whence it gradually inclines to the outer wall of the jaw.

The whole ascending ramus of the jaw consists of a
very thin plate of bone; it is slightly concave on the inner side, and
the inferior margin of the produced angle inclines inwards, as in the
Mylodon and Sloth; it is impressed on the outer side with two shallow
depressions, and two parallel ridges, both following the gentle
curvature of the part. There is a foramen on the outer side of the
ramus at the anterior part of the base of the coronoid process
corresponding with that in the lower jaw of the Mylodon, but the
longitudinal channel which runs along the outer side of the alveolar
processes is wanting, and the expansion at the base of those processes
is more sudden and relatively greater; the general correspondence,
however, between these lower jaws is such as would lead to the idea
that they belonged to animals of the same genus, were it not that the
teeth present modifications of form in the Scelidothere, as distinct
from those of the Mylodon, as are any of the minor dental differences
on which genera or sub-genera of existing Mammalia are founded in the
present state of Zoological Classification.

To make this distinction more readily intelligible,
I have given a view of the transverse section of the teeth in the right
ramus of the lower jaw (fig. 4, Pl. XXIII.), corresponding with that of
the Mylodon Darwinii, (Pl. XVII., fig. 5). In the present

sub-genus the antero-posterior extent of the four
alveoli of the lower jaw nearly equals four inches, and is relatively
greater than in the Mylodon, although the teeth are placed closer
together; this is owing to their greater relative size. The first molar
tooth presents the simplest form; its transverse section is a
compressed inequilateral triangle with the angles rounded off; the
longest diameter of this section which is parallel with the inner
alveolar border is eleven lines, the transverse diameter almost six
lines; the base or broadest side of the triangle is turned inwards,
and is slightly concave; the two smaller sides are also slightly
concave.

The second molar is placed more obliquely in the
jaw; the long axis of its transverse section intersects at an acute
angle
that of the jaw itself; the transverse section presents a compressed or
oblong form, with the larger end next the outer side, and the smaller
end next the inner side of the jaw; this end is simply rounded, but
the outer end presents a sinuosity, corresponding to a broad groove
which traverses the whole length of the outer side of the tooth; the
anterior, which corresponds to the internal side or base of the
transverse section of the preceding molar, is slightly concave.

The third molar has nearly the same form and
relative position as the preceding; the long diameter of the transverse
section is, in both, ten lines and a half; the principal transverse
diameter is, in the second molar five lines, in the third nearly six;
the difference of form observable in these as compared with the two
middle grinders of the Mylodon is well marked; in the latter these
teeth are impressed with a longitudinal groove on their inner sides;
in the Scelidothere they have a similar impression along their outer
but not along the inner side.

In the last molar the resemblance is much closer,
and the modification of form by which it differs from the preceding
ones is of the same kind; the transverse section gives an irregular
oblong figure with its axis nearly parallel with that of the jaw, and
constricted at the middle by sinuosities produced by two wide channels
which traverse longitudinally, one the outer, the other the inner side
of the tooth; the latter groove is much wider and shallower in the
Scelidothere than in the Mylodon. The two lobes produced by these
grooves are more equal in Scelidothere; the anterior one is concave on
its anterior surface instead of convex as in the Mylodon; the posterior
one is more compressed; the longitudinal or antero-posterior diameter
of the transverse section of this tooth is one inch five lines; the
greatest transverse diameter is nine lines; the diameter of the
isthmus joining the lobes is three lines and a half; the entire length
of this tooth is three inches three lines.*

* It requires little stretch of imagination to
conceive that this more complex posterior tooth (Pl. XXIII, fig. 4, 4)
in the lower jaw is the representative of the two smaller posterior
teeth (ib. fig. 3, 4, and 5) of the upper jaw conjoined.

Of this part of the skeleton of the Scelidothere,
Mr. Darwin's specimen includes, as is represented in Plate XX., the
cervical, part of the dorsal, and the sacral series of vertebræ in a
more or less perfect condition.

The cervical vertebræ present the ordinary
mammalian number, seven, and are free, or so articulated as to have
permitted reciprocal movement upon each other. Their transverse
processes are perforated as usual for the vertebral arteries. These
processes in the atlas are remarkable for their great breadth, length,
and thickness; and indicate the muscular forces which must have worked
the head upon the spine to have been very powerful. The axis is
provided with a robust 'processus dentatus,' having a base equal in
breadth to the body of the axis itself; and a smooth articular
convexity on the side of the apex on which the ring of the atlas
rotated. The line of union between the axis and its characteristic
process, which here resembles the body of an abortive vertebra, is very
distinct. The transverse processes of the vertebra dentata are
comparatively feeble, but this condition is amply compensated for by
the great development of the spinous process. (Pl. XXIV. fig. 1.) This
process is bent backwards at nearly a right angle, overlaps with its
reflected extremity the spine of the third cervical vertebra, and rests
by its base, on the under part of which are the posterior articular
surfaces, upon the broad and strong anterior oblique processes of the
third vertebra.

The third, fourth, fifth, and sixth cervical
vertebræ have moderately developed and pointed spinous processes:
their transverse processes are broad, and extend obliquely backwards,
and slightly overlap each other. On the under part of the transverse
process of the sixth cervical vertebra there is the fractured base of
what I conjecture to have been an expanded aliform plate, analogous to
that observable in the corresponding vertebra of the Orycterope. The
seventh cervical vertebra has part of the articular depression for the
head of the first rib upon each side of its body: the transverse
process is feebly developed, but the spine is double the height and
size of those of the preceding vertebræ.

The spinous process of the first dorsal vertebra in
like manner rises to twice the height of the preceding spine of the
seventh cervical, and preserves an equal antero-posterior diameter from
its base to its summit, which is thick and slightly bent backwards:
four or five succeeding dorsal vertebræ give evidence of having been
surmounted by spines of equal height and strength. The transverse
processes of these dorsal vertebræ present bold concavities on their
inferior part for the reception of the tubercles of the ribs, and they
gradually ascend upon the base of the spines as the vertebræ are
placed further back, so as to increase the expansiveness

of the chest. The state of the fossil did not afford
further information as to the condition of this part of the vertebral
column, but the parts which have been preserved are precisely those
from which the most interesting inferences as to the affinities and
habits of the extinct quadruped can be deduced.

Whether the Megatherium be most nearly allied to the
tribes of the Sloth or Armadillo has been a question under recent
discussion, and, as a corollary of this problem, whether its habits
were those of a scansorial or of a fossorial quadruped. For, strange as
it may appear at first sight, there have not been wanting arguments,
and those urged by an anatomist to whom we owe much novel and
interesting information respecting the extinct Edentata, in support of
the belief that the Megatherium, gigantic and ponderous as must have
been its frame, actually climbed trees like a Sloth, and had claws and
feet organised for prehensile actions, and not in accordance with that
type by which they are usually adapted for digging up the soil.*

Now, in whatever degree the Megatherium may be
involved in this question, the smaller Megatherioid species at present
under consideration must be at least equally implicated in it. In the
adaptation of the frame of a mammiferous quadruped for especial and
peculiar actions and modes of life, such as for climbing and living in
trees, or for burrowing and seeking concealment in the earth, not only
the immediate instruments, as the feet, are modified, but the whole of
the osseous and muscular fabric is more or less impressed with
corresponding adaptations, whilst at the same time these special
adjustments are invariably subordinated to the type of organization
which characterizes the group.

The type of the order Bruta or Edentata is well-marked; one or more claws of unusual length and strength,
characterize the fore-feet and sometimes the hind-feet in every genus,
and the term 'Macronykia' would more aptly designate them than the
term which Cuvier substituted for the good old Linnæan appellation.
The uniform absence of true roots to the teeth, where these are
present, is another general character; the skeleton exhibits many
well-marked peculiarities common to the whole order; while at the same
time it is modified in various modes and degrees in accordance with the
peculiar habits and exigencies of the species.

One of the regions of the skeleton which manifests
adaptive modifications of this kind in the most remarkable degree is
the cervical division of the vertebral column. In one edentate species
it is lengthened out by two additional vertebræ more than in any other
mammal; in another it is reduced by anchylosis to as great an extent
below the regular number of moveable pieces: and these, the two most
opposite conditions of the cervical vertebræ which are to be met with
in the mammiferous class are related to equally diverse and opposite
habits of life.

With respect to the Ai, or three-toed
Sloth, "an animal, great part of whose life, when not engaged in
eating, is spent in sleeping on trees,—an easy attitude for repose is
most essential to its comfortable existence; and accordingly we find,
that the auxiliary vertebræ at the base of the neck contribute to
produce that flexibility of this organ which allows the head of the
animal to incline forwards and rest upon its bosom." Dr. Buckland, from
whose Paper on the "Adaptation of the Structure of the Sloths to their
peculiar Mode of Life,"* the preceding judicious physiological remark
is quoted, adduces the authority of Mr. Burchell in proof that the
Sloth can in a remarkable manner and with great facility twist its head
quite round, and look in the face of a person standing directly behind
it, while at the same time the body and limbs remain unmoved. A single
glance at the length and slenderness of the cervical region of the
spine, and of the feeble condition of the transverse and spinous
processes in the vertebræ composing that part of the skeleton of the
Sloth, is enough to show its adaptation to increase the rotatory motion
and flexibility of the neck.

In describing the skeleton of a species of Armadillo (Dasypus 6-cinctus, Linn.)† I was led in like manner to point
out the subserviency of the peculiarities of the cervical vertebræ to
the habits and mode of life of that animal; observing that the
"anchylosis of the cervical vertebræ obtains in the Cetacea,
as
well as in the genus Dasypus, and that as in the aquatic
order
this firm connexion of the cervical vertebræ assists materially in
enabling the head to overcome the resistance of the dense fluid through
which they perpetually move, so in the Armadillos a like advantage may
be derived from this structure during the act of displacing the denser
material in which they excavate their retreats."‡

Having in view these well-marked examples of the
subserviency of the structure of the bones of the neck to the habits of
existing species of the order Bruta, I proceeded to
investigate the structure of the corresponding part of the skeleton in
the Scelidotherium, hoping thereby to gain a new and useful
element in the determination of the problem at present under
discussion, as to the affinities and habits of the extinct Megatherioid
quadrupeds.

The fossil, in its original state, yielded a view of so much
of the
anterior part of the bodies of the cervical vertebræ as proved that
they were neither so numerous as in the Sloth, nor anchylosed together
as in the Armadillos: after a long and careful chiselling at the hard
matrix in which they were imbedded, the trans-

* Linn. Trans, vol. xvii. (1833) p. 17.

† Zool. Proceedings, 1832, p. 134.

‡ The anterior prolongation of the sternum
in front of the neck and the corresponding anterior position of the
clavicles and scapulæ occasions a transference of such a proportion of
the moving powers of the head from the cervical vertebræ to these
bones in the mole, as renders any modifications of these vertebræ,
like those in the Armadillo, uncalled for.

verse and spinous processes were exposed to view, as
they are represented in Plates XX. and XXIV. The description of these
processes has already been given.

On comparing the cervical vertebræ of the
Scelidotherium with those of the existing Bruta, the closest
resemblance to them was found in the skeleton of the Orycterope. Now
this quadruped, though not so rapid a burrower, or so strictly a
subterranean species as the Armadillos, participates, nevertheless, to
a certain extent, in their fossorial habits, and is closely allied to
them in general structure: it differs from them, indeed, mainly in a
modification of the dental system, in the absence of dermal armour, and
of anchylosis of the cervical vertebræ. But the advantages which, as a
burrower, it would have derived from the latter structure, are
compensated for by the shortness of the cervical vertebræ, and by the
great development and imbricated or interlocking co-adaptation of the
transverse and anterior spinous processes of the cervical vertebræ.
The analogous quadruped in the South American Continent—the great
ant-eater (myrmecophaga jubata) which uses its powerful
compressed fossorial claws for breaking through the hard walls of the
habitations of its insect prey, but which does not excavate a
subterraneous retreat for itself, presents the cervical vertebræ of a
more elongated form, and without that development of the spinous and
transverse processes which tend to fix the neck and increase the size
of the muscles which move the head: and, if we could conceive that its
fore-feet were employed to scratch up vegetable roots, instead of
disinterring termites, there would be no reason to expect any
modification of the cervical vertebræ as a direct consequence of such
a difference in the application of its fossorial extremities: when,
therefore, we find that the cervical vertebræ do actually differ in
two myrmecophagous species, to the extent observable in the Cape and
South American ant-eaters, we arrive legitimately at the conclusion
that such difference relates to fossorial habits of the one species, in
which habits the other does not participate.

Now, therefore, if this conclusion be just in regard
to the Orycterope, it must bear with more force upon the question of
the habits of the Scelidotherium as the mechanism for strengthening the
connection of cervical vertebræ, and for augmenting the surface of
attachment of the muscles which worked the head and neck, is more
strongly wrought out in that extinct species.

The great size and strength of the spinous process
of the dentata, and the mode in which it is interlocked with the
spinous and oblique processes of the third cervical, together with the
imbricated disposition of the transverse processes of this and the
succeeding vertebræ, and the remarkable height of the dorsal spines,
all combine to indicate in a very striking manner, if not to
demonstrate, that the conical head of the present species, which is
comparatively small and slender, and

for its own mere support requiring therefore no such
mechanism, was used in aid of the fossorial actions of the extremities.

As the cervical vertebræ of the Megatherium have
their processes comparatively weaker than in the Scelidotherium, and
the anterior dorsal spines are relatively shorter, it may be concluded,
that whatever were the extent or nature of the fossorial labours of the
enormous claws with which it was provided, the head did not co-operate
with the digging implements in their especial task in the same degree
as in the Scelidothere and Orycterope. At the same time there is no
modification of the cervical region of the spine of the Megathere
corresponding with those which we have seen to be subservient to the
arboreal habits of the sloth, a remark which will not be deemed
superfluous by those who have perused the acute observations and
arguments adduced by M. Lund in favour of the scansorial character of
the extremities of the Megatherium and Megalonyx.

The fragments of the dorsal vertebræ and ribs of
the Scelidotherium, which are figured in Plate XX, offer no
modifications which need detain our attention; they closely conform,
excepting in the greater relative height of the anterior dorsal spines,
already noticed, with the Megatherioid type. The sacrum manifests in
its vast expanse, the great development of the posterior transverse
processes to join the ischium, the capacious medullary cavity, and wide
nervous foramina, a like conformity with the Megatherium, and a
corresponding harmony with the disproportionate bulk of the hind legs.

BONES OF THE EXTREMITIES.

The Scapula in its double spine, the osseous arch
formed by the confluence of the acromion with the coracoid process, and
the substitution of a distinct foramen for the suprascapular notch,
agrees with that of the Megatherium: but the span of the acromial arch
is relatively wider, and the surface for the articulation of the
clavicle is better marked. This articular surface, which is distinctly
shewn upon the acromion of both the scapulæ in Pl. XX. is the more
interesting, as being the only evidence of the clavicle of the
Scelidothere which we at present possess; but it is enough to prove
that this quadruped enjoyed all the advantages in the actions of the
fore-extremity, which arise out of the additional fixation of the
shoulder-joint afforded by the clavicle—a bone which the extinct
Megatherioids are the largest of the mammiferous class to possess in a
completely developed state. The form, position, and aspect of the
glenoid cavity for the humerus closely correspond with the condition of
the same part in the Megatherium. The limits of the acromial and
coronoid portions of the arch were still defineable in the

present skeleton, which indicates the nonage of the
individual in the unanchylosed condition of most of the epiphyses.

In regard to the presence of a clavicle in the
Megalonyx M. Lund has deduced certain conclusions, which, if well
founded, would be equally applicable to the present allied species, and
to the great Megatherium. I am induced, therefore, to offer a few
physiological observations on that bone, which appear to me to lead to
a more correct interpretation of its uses and relations in the great
mammiferous animals now under consideration.

When the anterior extremities in mammalia are used
simply for the purpose of progressive motion on dry land, as in the
Pachyderms and Ruminants, or in water, as in the Cetaceans, there is no
clavicle; this bone is introduced between the sternum and acromion, in
order to give firmness and fixity to the shoulder-joint when the
fore-leg is to discharge some other office than that of locomotion. In
these cases, however, the clavicle exists in various degrees of
development, and even its rudiment may be dispensed with in some of the
actions which require a considerable extent of lateral or outward
motion, and of freedom of rotation of the fore-limb. When, therefore,
we find the clavicle fully developed in the skeleton of an extinct
mammiferous animal, and so placed as to give the humeral articulation
all the benefit of this additional mechanism, we may confidently expect
that it will afford an insight into the habits and mode of life of such
extinct species. M. Lund* has argued from the clavicle of the
Megalonyx, that it climbed like a Sloth. "Animals," says Sir C. Bell,†
"which fly or dig, or climb, as Bats, Moles, Porcupines, Squirrels,
Ant-eaters, Armadilloes, and Sloths, have this bone; for in them, a
lateral or outward motion is required." But in regard to the present
problem, we have to enquire whether the clavicle manifests any
modifications of form, of strength, or development in relation to the
special differences of these several actions, with which its presence
is asserted to be associated?

In mammals which fly, the clavicle is always complete: the
rabbit,
the fox, and the badger are instances of burrowing animals in which the
clavicle is absent or rudimental. The presence of a perfect clavicle is
not more constant in climbing quadrupeds. The Ai, for example, has an
incomplete clavicle, which is attached to the acromion process, and
terminates in a point about one-fourth of the distance between the
acromion and the top of the sternum, to which the clavicular style is
attached by a long slender ligament: the advantage, therefore, which a
perfect clavicle affords in the fixation of the shoulder-joint, is lost
to this climber par excellence. Again, the Bears, which are
the bulkiest quadrupeds that are gifted with the faculty of climbing,
and this in so perfect a degree that the Sun-bears of the Eastern
Tropics may be termed arboreal animals,—these scan-

sorial quadrupeds are destitute of even the smallest rudiment of a
clavicle, as I have ascertained by repeated careful dissection.

Since, therefore, a clavicle in any degree of
development is not essential to a climbing quadruped, we must seek for
some other relation and use of that remarkably strong, and perfect
bone, as it exists in the Megathere, Megalonyx, and Scelidothere. The
absence of 'dentes primores' or of anterior or incisive teeth in these
quadrupeds at once sets aside any idea of its connection with an action
of the fore extremities, very common in the mammals which possess
clavicles, viz., that of carrying the food to the mouth, and holding it
there to be gnawed by the teeth. Flying is of course out of the
question, although our surprise would hardly be less at seeing a beast
as bulky as an elephant climbing a tree, than it would be to witness it
moving through the air. If now we restrict our comparison to the
relations of the clavicle in that order of Mammalia to which the
extinct species in question belonged, we shall see that it is most
constant, strongest, and most complete in those species which make most
use of their strong and long claws in displacing the earth, as the
Armadilloes and Orycteropus: and, as the clavicle is incomplete in one
climbing Edental, we are naturally led to conclude that its perfect
development in an extinct species must have been associated with uses
and relations analogous to those with which it coexists in other genera
of the same order. Thus it will be seen, that, in rejecting the
conclusion drawn by M. Lund from the presence of a clavicle, I concur
in the opinion expressed by Dr. Buckland* that the Megatherium—and with
it the Megalonyx and Scelidotherium—had the shoulder-joint strengthened
by the clavicle, in reference to the office of the fore-arm, as an
instrument to be employed in digging roots out of the ground. Not,
however, that these gigantic quadrupeds fed on roots, but rather, as
the structure of the teeth would show, on the foliage of the trees
uprooted by the agency of this powerful mechanism of the fore-legs, and
of the otherwise unintelligible colossal strength of the haunches,
hind-legs, and tail.

The humerus presents a large convex oval head, on each side of which
is a tuberosity for the implantation of the supra- and sub-scapular
muscles: these tuberosities do not rise above the articular convexity,
so as to restrict the movements of the shoulder-joint, as in the Horse
and Ruminants, but exhibit a structure and disposition conformable to
those which characterize the proximal extremity of the humerus in other
mammalia which enjoy rotatory movements of the upper or fore-limb. The
tuberosities are, however, relatively more developed, and give greater
breadth to the proximal end of the humerus in the Scelidothere than in
the Megathere. The distal end of the humerus, although mutilated,
clearly indicates that it had the same characteristic breadth of the
external and internal

condyles, as in the Megatherium. In fig. 1. Pl. XXV.
which gives a front view of the left humerus, the broad internal
condyle, with its extremity broken off, is seen projecting to the left
hand; both in this figure and in fig. 2. in which the internal side of
the humerus is turned towards the observer, the wide groove, with its
two osseous boundaries, is shewn, which plainly indicates that the left
condyle was perforated for the direct passage of the artery or median
nerve, or of both, to the fore-arm. The groove for the musculo-spiral
nerve on the outer side of the humerus is over-arched at its upper part
by a strong obtuse process; which is comparatively less developed in
the Megatherium. The trochlear or inferior articular surface of the
humerus presents, as in the Megatherium, two well-marked convexities,
with an intervening concavity: this indication of the rotatory power
of the fore-leg is confirmed by the form of the head of the radius.

In Pl. XXV. fig. 4. a view is given of this
articular surface: it presents the form of a subcircular gentle
concavity, which plays upon the outer convexity of the humeral
articular surface: immediately below the upper concavity the radius
presents a lateral smooth convex surface, which rotates upon a small
concavity on the ulna, analogous to the 'lesser semilunar,' in human
anatomy, in which the mechanism for rotation, so far as the upper joint
of the radius is concerned, is not more elaborately wrought out than in
the present extinct edentate quadruped. The radius expands as it
proceeds to the elbow-joint, where it attains a breadth indicative of
the great power and size of the unguiculate paw, of which it may be
called the stem, and to the movements of which it served as the pivot.

All the bones of the fore-limb just described—the
scapula, the humerus, and the radius,—indicate by the bold features and
projections of the muscular ridges and tubercles the prodigious force
which was concentrated upon the actions of the fore-paw, and the ulna,
in its broad and high olecranon (of which a side-view is given in fig.
2. Pl. XXV.) gives corresponding evidence. The great semilunar
concavity is traversed by a sub-median smooth ridge, which plays upon
the interspace of the two humeral convexities. The body of the bone is
subcompressed, straight, and diminishes in size as it approaches the
carpal joint: the immediate articulating surfaces are wanting in both
the radius and ulna, the epiphysial distal extremities having become
detached from their respective diaphyses.

Of the terminal segment of the locomotive
extremities, the only evidence among the remains of the skeleton of the
Scelidothere is the ungueal phalanx figured at Pl. XXVII. 3, 4, and 5;
but as it is uncertain whether it belong to the fore or hind-foot, it
will be described after the other bones of the extremities have been
noticed.

Of these bones the femur is the most remarkable,
both for its great proportional size, and its extreme breadth, as
compared with its length or thick-

ness: but in all these circumstances the affinity
of the Scelidothere with the Megathere is prominently brought into
view. There is no other known quadruped with which the Scelidothere so
closely corresponds in this respect. In proceeding, however, to compare
together the thigh-bones of these two extinct quadrupeds, several
differences present themselves, which are worthy of notice: of these
the first is the presence in the Scelidothere of a depression for a
'ligamentum teres' on the back part of the head of the femur, near its
junction with the neck of the bone: this is shewn in the posterior view
of the femur given in Pl. XX. The head itself forms a pretty regular
hemisphere: the great trochanter does not rise so high as in the
Megatherium, but, relatively, it emulates it in breadth: the small
trochanter is proportionally more developed: the external contour of
the shaft of the femur is straighter in the Scelidothere than in the
Megathere, and the shaft itself is less bowed forwards at that part.
The articular condyles occupy a relatively smaller space upon the
distal extremity of the femur in the Scelidothere, and they differ more
strikingly from those of the Megathere, in being continued one into the
other: the rotular surface, for example, which is shewn in fig. 5. Pl.
XXV. is formed by both condyles, while in the Megatherium it is a
continuation exclusively of the external articular surface.

The patella, which works upon the above-mentioned
surface, is a thick strong ovate bone, with the smaller end downwards:
rough and convex externally, smooth on the internal surface, which is
concave in the vertical and convex in the transverse directions.

Of the bones of the leg only the proximal end of the
tibia is preserved; but this is valuable, as shewing another
well-marked difference between the Scelidothere and Megathere; for
whereas in the latter the fibula is anchylosed with the tibia, this
bone, in the Scelidothere, presents a smooth flat oval articular
surface, which is shewn in fig. 2. Pl. XXVII. below the outer part of
the head of the bone; from the size and appearance of which, I infer,
that the fibula would not have become confluent with the tibia, even in
the mature and full-grown animal.

The relative length of the fore and hind extremities
cannot be precisely determined from the present imperfect skeleton of
the Scelidothere; but there is good evidence for believing, that the
fore extremity was the shortest. The humerus is shorter than the femur
by one-ninth part of the latter bone; and the radius, which wants only
the distal epiphysis, must have been shorter than the humerus. Now the
relative development of the fore and hind legs is one of the points to
be taken into consideration in an attempt to determine the habits and
nature of an extinct mammal.

In climbing animals the prehensile power is more essential to the
hinder than to the fore parts or extremities. In the leech the
principal sucker is in the tail;

and higher organized climbers, in like manner,
depend mainly on their posterior claspers in descending trees, and hold
on by means of them whilst selecting the place for the next application
of those at the fore part of the body, whether their place be supplied
by the beak, as in the Maccaws, or the fore-feet or hands in the
Mammalia.

But, although we perceive the hinder limbs to be the
last to lose the advantageous structure of the hand in the Quadrumanous
species, and notwithstanding that the tail is for this purpose
sometimes specially organized to serve as a prehensile instrument, yet
we find that the power of grasping the branches of trees by either legs
or tail is never maintained at the expense of undue bulk and weight of
those organs. On the contrary, as the fore-limbs are the main
instruments in the active exertions of climbing, so they are the
strongest as well as the longest in all the best climbers, and the
weight of the body which they have to drag along is diminished by
dwarfish proportions of the hinder limbs, as in the Orangs and the
Sloths.

Can those huge quadrupeds have been destined to
climb that had the pelvis and hinder extremities more ponderous and
bulky in proportion to the fore-parts of the body than in any other
known existing or extinct vertebrate animals?

M. Lund argues for the scansorial character of the
Megalonyx, because its anterior extremities are longer than the
posterior ones; but if they somewhat exceed the hind-legs in length,
how vastly inferior are they in respect of their breadth and thickness.
The prehensile faculty of the hinder limbs of the best climbers, as the
Sloths, Orangs, and Chameleons is by no means dependent on the superior
mass of muscle and bone which enters into their conformation, but is
associated with the very reverse conditions.

It is impossible to survey the discrepancy of size
between the femur and the humerus of the Scelidothere, as exhibited in
Pl. XX., without a conviction that it relates to other habits than
those of climbing trees. The expanse of the sacrum, the evidence of the
muscular masses employed in working the hind legs and tail, which is
afforded by the capacity of the cavity lodging the part of the spinal
marrow from which the nerves of those muscles were derived, both
indicate the actions of the hind-legs and tail to have been more
powerful and energetic than would be required for mere prehension: and
the association of hinder extremities so remarkable for their bulk,
with a long and powerful tail, forbids my yielding assent to the
speculation set forth by M. Lund, as to the prehensile character of the
tail of the Megalonyx.

Astragalus.—In the examination of this
characteristic bone I have kept in view the question of the habits of
the Megatherioid quadrupeds in general, and the especial affinities of
the Scelidotherium, in illustration of which I shall notice at

the same time the peculiarities of the astragalus of
the Sloth, Megatherium and Armadillo.*

The upper articular surface of the astragalus of the
Scelidotherium (Pl. XXVI. fig. 4.), presents, in its transverse
contour, two convex pulleys, a and b, and an
intermediate concavity, forming one continuous articular surface. The
external or fibular trochlea (a) is strictly speaking convex
only at its posterior part, the upper surface gradually narrowing to a
ridge, as it advances forwards, from which, the inner and
outer parts slope away at an angle of 35°.

The tibial† convexity (b) is more
regular
and less elevated, it has only half the antero-posterior extent of the
outer pulley; its marginal contour forms an obtuse angle at the inner
side.

In the Megatherium the upper articular surface of
the astragalus is also divided into two trochleæ, of which the one on
the fibular side (fig. 3, a), is of much greater relative
size
and extent than the tibial one (b), and is raised nearly four
inches above the level of the latter, although in the oblique position
in which the bone is naturally placed in the skeleton, the highest part
of each convexity is on the same level The fibular trochlea differs
also from that in the Scelidothere in being regularly convex in the
transverse as well as the antero-posterior direction. The tibial
convexity resembles that in the Scelidothere, save in its smaller
relative size; its internal margin likewise forms an angular projection
below the internal malleolus.

The upper surface of the astragalus of the Mylodon,
or Megalonyx(?) (Pl. XXVIII. fig. 5.),‡
differs from that in the Megatherium in having a narrower fibular
trochlear ridge.

The astragalus of the Ai (Bradypus tridactylus) differs
widely from that of either the Megathere, Mylodon (?) or Scelidothere
in having a conical cavity on the upper surface, in place of the
fibular convexity, in which concavity the distal end of the fibula
rotates like a pivot. This mechanism is closely related to the
scansorial uses of the inwardly inflected foot of the Sloth.

If the astragalus of an Armadillo§ were placed side by side with
that of the

* Dasypus 6-cinctus, L., is the
species of
which I have the astragalus separate, so as to be able to follow out
the comparison.

† In distinguishing these trochleæ as fibular and
tibial, it is to be understood that the terms relate only to aspects
corresponding to the position of those bones, and not that the fibula
is articulated to the whole of the trochlea so called: it probably
rested only upon the outer facet in the Scelidothere.

‡ This astragalus was found at Santa Fé, in Entre
Rios, associated
with the remains of the Mastodon and Toxodon; but from its size and
form I entertain little doubt that it belonged to a Megatherioid
quadruped as large as the Mylodon or Megalonyx. The brief allusion to
the astragalus of the Megalonyx in M. Lund's Memoir does not afford the
means of determining with certainty this point.

§ See the figures of this bone, given by Cuvier in
Pl. x. and xi.
Ossemens Fossiles, vol. v. part i.

Megathere, it would be very difficult to determine
the analogous parts, especially of the upper surface, unless guided by
the intermediate structure presented by the Scelidothere. The upper
surface of this bone, in the Armadillo, is, however, divided into
two transversely convex trochleæ, separated by a much wider
transversely concave surface. The fibular trochlea resembles that of
the Scelidothere in having its upper and outer facets sloping away at
an acute angle, but without meeting at a ridge anteriorly; this surface
is not more raised above the tibial trochlea than in the Scelidothere.

The inner trochlea differs from that of the
Scelidothere in having a greater relative antero-posterior extent, and
in forming, in place of an uniform convex surface, a trochlea similar
in structure to that on the outer side. The extent of rough surface on
the upper part of the astragalus intervening between the articular
surface for the bones of the leg, and that for the scaphoides is
extremely small in the Megathere and Mylodon (?); it is relatively
greater in the Scelidothere; it is still more extensive in the
Armadillo; but is the longest in the Sloth. The anterior extremity of
the astragalus which is entirely occupied by the scaphoid articular
surface is very peculiar in the Scelidothere (Pl. XXVI. fig. 2.): it
presents one convex and two concave facets, which, however, form part
of one continuous articular surface: the convex facet forms the
internal part of the surface, and presents a rhomboidal form with the
long axis vertical. The concave facets (c and d)
are
extended transversely and placed one above the other; they are slightly
concave in the transverse, and nearly flat in the vertical directions.

In the Megatherium (fig. 1.) the scaphoid surface of
the astragalus is divided only into one concave and one convex portion,
both continuous with each other: the concave facet (c)
corresponds
with
the upper concavity in the Scelidothere, but is a pretty uniform
subcircular depression, fourteen lines in depth: the convex facet, d,
is continued across the whole breadth of the under part of the
scaphoid surface and corresponds with both the inner convex, and lower
concave surfaces of the scaphoid articulation in the Scelidothere.

In the Mylodon (?) (Pl. XXVIII. fig. 3.), the
articular facet, corresponding with that marked (c) in the
astragali of the Megathere and Scelidothere, is simply flattened,
instead of being concave; the rest of the scaphoid surface corresponds
with that in the Megatherium.

In the Armadillo the scaphoid articular surface is undivided and
wholly convex: in this part of the astragalus, therefore, we find the
Scelidothere deviating from the Armadillo further than does the
Megathere; while the Mylodon or Megalonyx (?) most resembles the
Armadillo in the configuration of this part of the astragalus.

If we compare the outer surfaces of the astragalus in these
quadrupeds,

we shall find, however, that the Scelidothere and
Armadillo closely agree: the outer facet of the fibular trochleæ,
above described, is continued in the Scelidothere (Pl. XXVIII. fig.
2.), upon the fibular side of the astragalus reaching nearly half-way
down the posterior part, and down nearly the whole of its anterior.

In the Armadillo, it extends over the whole of the
anterior part of the outer side of the astragalus. In both animals the
lower boundary of this articular surface describes a strong sigmoid
curve.

In the Megatherium (Pl. XXVIII. fig. 1), the
corresponding surface for the fibular malleolus on the outer side of
the astragalus is formed by a comparatively very small semicircular
flattened facet, which by its roughness indicates that the end of the
fibula was attached to it by ligamentous substance, and that the
synovial bag was not continued upon that surface as in the Scelidothere
and Armadillo.

In the Mylodon (?) (Pl. XXVIII. fig. 4),
even this rough facet is wanting and the fibular trochlea is bounded by
the angle which divides the upper from the outer surface of the
astragalus.

Turning now our attention to the under surface of
the astragalus, we observe that it presents in the Scelidothere (Pl.
XXVI. fig. 6), an irregular quadrate form, having the outer side
occupied by an elongated sub-ovate articular facet, e, for
the
calcaneum, bounded externally by a sharp edge, with its long axis and
its greatest concavity in the antero-posterior direction, and slightly
convex from side to side: a second calcaneal articular surface (f)
is
situated at the inner and anterior angle; it is oblong and nearly flat;
is continuous with the inferior concave facet of the scaphoid
articulation, but is divided from the convex facet by a groove: the two
calcaneal articulations are separated by a deep and rough depression,
traversing the under surface of the astragalus diagonally, and
increasing in breadth towards the posterior and internal angle. The
inner side of the astragalus presents a convex protuberance.

The correspondence between the astragalus of the Scelidothere and
Megathere is best seen at the under surface of the bone: in both the
two calcaneal articulations are separated by the diagonal depression,
and the internal and anterior surface is continuous with the scaphoid
articulation. In the Megathere, however, in consequence of the absence
of the inferior concavity which characterizes the Scelidothere, the
anterior calcaneal facet (f) appears as a more direct backward
continuation of the scaphoidal surface; but they are divided by a more
marked angle than is represented in the figure (fig. 5, Pl. XXVI.). The
posterior and outer calcaneal surface in the Megathere (e) is
broader
in proportion to its length, continued further upwards upon the outward
surface, is consequently more convex in the transverse direction, and
is not bounded externally by so sharp and prominent a ridge as in the

Scelidothere. The protuberance from the inner
surface of the astragalus is more compressed laterally in the Megathere
than in the Scelidothere. The correspondence between the astragali of
the Mylodon (?) (Pl. XXVIII. fig. 6) and Megathere in the conformation
of the under surface is so close, that the few differences which exist
will be sufficiently appreciated by an inspection of the figures.

In the Armadillo the astragalus, in consequence of
the greater production of its anterior part, presents more of an
angular than a quadrate figure; and the scaphoid articular surface,
being proportionally carried forwards, is altogether separated from the
anterior calcaneal surface. The posterior and inner calcaneal surface
resembles that in the Scelidothere, but is less inclined upwards; and
is continuous with the posterior part of the tibial articular surface.

Thus the astragalus in the structure of its two most
important articulations, viz. that which receives the superincumbent
weight from the leg, and that which transmits it to the heel, presents
a closer correspondence in the Scelidothere with that of the Dasypus,
than with that of the Megathere or Mylodon.

The ungueal phalanx of the Scelidothere before
alluded to, is represented of the natural size in Pl. XXVII. The
side-view, fig. 3. shows the position of the articular surface on the
proximal end, sloping obliquely towards the under surface, and
overtopped by an obtuse protuberance, calculated to impede any upward
retraction of the claw: the present joint, in fact, illustrates in
every particular the argument by which Cuvier established the true
affinities of the allied extinct genus Megalonyx.*

The present phalanx is, however, less compressed,
and less incurved than those of the Megalonyx, which have been hitherto
described; but it more resembles in these proportions one of the
smaller, and presumed hinder, ungueal phalanges of the Megatherium. The
upper and lateral parts of the bone are rounded, and it gradually
tapers to the apex, which is broken off. The osseous sheath for the
claw is developed only at the under part of the bone: it presents the
form of a thick flat plate of bone, with the margin very regularly and
obliquely bevelled off, and having a vertical process of bone attached
lengthwise to the middle of its under surface This process must have
served for the insertion of a very powerful flexor tendon. The figures
of this bone preclude the necessity of any further verbal description.

M. Lund lays most stress upon the argument founded on the inward
inflection of the sole of the foot in the Megalonyx, and appeals with
greatest confidence to this structure in support of his hypothesis of
the scansorial habits of that extinct Edental.†

* Ossemens Fossiles, vol. v. part i. p. 163.

† For the translation of the following passage, and
of others
alluded to in the present work, from the original Danish Memoir of M.
Lund, loc. cit., I am much indebted to the Rev. W. Bilton, M.A. &c.
&c.:—

It is quite true that the Quadrumana derive
advantage from this position of the foot in climbing trees, and that it
is carried to excess in the Sloths, which can only apply the outer edge
of the foot to the ground. But we may ask, was the inversion of the
sole of the foot actually carried to such an extent in the Megalonyx? And, admitting its existence in an inferior degree, is it then
conclusive as to the scansorial habits of that species?

M. Lund expressly states that it is produced by a
different structure and arrangement of the tarsal bones, from that
which exists in the Sloth, but he does not specify the nature of this
difference.

If the astragalus, which I have referred with doubt to the Megalonyx,
do not actually belong to that genus, it is evidently part of a
very closely allied species. Now this astragalus, as we have seen,
resembles most closely that of the Megatherium; and since we may infer
that the calcaneum, scaphoides, and cuboides had a like correspondence,
the inclination of the sole of the foot inwards must have been very
slight, as I have determined from examination of the structure and
co-adaptation of those bones in the incomplete skeleton of the
Megatherium in the London College of Surgeons. Such an inclination of
the foot may be conceived to have facilitated the bending of the long
claws upon the sole, during the ordinary progressive movements of the
animal, but it is quite insufficient to justify the conclusion, that it
related to an application of the hind feet for the purposes of climbing.

It is not without interest again to call to mind the deviation of
the
structure

"Thus in every point of comparison we have
instituted between the organization of burrowers and climbers; we have
seen that the Megalonyx constantly differs from the former and
resembles the latter; but the point to which I last alluded (the
obliquity of foot), I consider to be quite decisive.

"There is one other point in its organization, which
is not quite without weight in reference to our present inquiry,—I mean
its unusually powerful tail. Now, it is certainly true that many
animals which are not climbers have a powerful tail, as e. g.
Armadillos, while the others that climb well, have none, as Sloths and
Apes. But when we find a remarkably powerful tail attached to an animal
that according to all probability was a climber, we are led to infer
that this organ must have served for that purpose: in other words, that
the Megalonyx was furnished with a prehensile tail.

"How far the Megatherium is to be considered in the
same light as the Megalonyx cannot be decided without an accurate and
scientific examination of its skeleton at Madrid. Pander and D'Alton do
not mention any distortion of the hind-foot, neither does their figure
exhibit any. It is nevertheless quite possible that such may exist, but
that it is disguised by the faulty manner in which the skeleton is put
up. It strikes me as little probable that two animals which agree so
well in the principal particulars of their organization should differ
so much in one of the most important. The Megatherium has been proved
by later discoveries to possess the same powerful tail as the
Megalonyx, and as it corresponds also with the latter entirely in the
conformation of its extremities, the same difficulties present
themselves against the supposition of its having been a burrower. But
if the Megatherium was really a climber, it must have had still more
occasion (on account of its greater size), for that peculiar
arrangement of the hind-feet which we have described in the Megalonyx."

of the astragalus of the Scelidothere from the
Megatherioid to the Dasypodoid type of structure. For if the
Megatherioid type of structure had really been one suitable to the
exigencies of climbing quadrupeds, it might have been expected to have
exhibited the scansorial modifications more decidedly, as the species
diminished in stature; but as regards the instructive bone of the
hind-foot, the modifications of which we have just been considering,
this is by no means the case.

DESCRIPTION OF A MUTILATED LOWER JAW OF THEMEGALONYX JEFFERSONII.

In the preceding section an astralagus was described, which was
regarded as belonging possibly to the same Edentate species as the jaw
figured and described, p. 69, Pl. XVIII. and XIX., under the name of Mylodon
Darwinii; but
the same correspondence,—that of relative size,—renders it equally
possible that this astragalus may belong to the species of Megalonyx to
which the lower jaw now under consideration appertains. There could be
no doubt, from its structure, that it was the astragalus of a gigantic
species of the order Bruta, and of the Megatherioid family,
and more nearly allied to the Megathere than is the Scelidothere, but
sufficiently distinct from both.

The lower jaw, figured in Pl. XXIX., is the only
fossil brought home by Mr. Darwin that could be confidently referred to
the genus Megalonyx; but the form of the tooth in place on
the
right side of the jaw fully justifies this determination. The jaw
itself is deeply and firmly imbedded in the matrix, so that only the
upper or alveolar border is visible. The coronoid and condyloid
processes are broken away, and the texture of the remaining part of the
jaw was too friable, and adhered too firmly to the surrounding matrix
to admit of more of its form being ascertained than is figured.

There were four molars on each side of this jaw; the
large oblique perforation near the fractured symphysis is the anterior
extremity of the wide dental canal. The forms of the alveoli are best
preserved in the right ramus: the first is the smallest, and seems to
have contained a tooth, of which the transverse section must have been
simply elliptical: the second tooth is likewise laterally compressed,
but the transverse section is ovate, the great end being turned
forwards: the third socket presents a corresponding form, but a larger
size: the fourth socket is too much mutilated to allow of a correct
opinion being formed as to the shape of the tooth which it once
contained. The natural size of the tooth

in situ, and of the adjoining socket, is
given in Pl. XXIX., fig. 2. The difference of form which the jaw of the
Megalonyx presents, as compared with that of the Mylodon, especially in
the greater recedence of the two horizontal rami from each other, will
be appreciated by comparing Pl. XVIII. with Pl. XXIX.

DESCRIPTION OF A FRAGMENT OF THE SKULL AND OF THE
TEETH OF THEMEGATHERIUM CUVIERI.

NOTWITHSTANDING the full, accurate, and elaborate
accounts of the skeleton of the Megatherium given by Brû,* Cuvier,†
Pander and D'Alton,‡; and Mr. Clift,§ the fragments of this most
gigantic of quadrupeds brought home by Mr. Darwin, possess much
interest, and have added, what could hardly have been anticipated,
important information as to the dental system, whereby an error in the
generic character of the Megatherium has been corrected.

The fragments here alluded to are portions of the
skull of three full-grown Megatheres: the most perfect part of which
affords a view of the posterior, and of part of the basal surface,
which regions of the cranium have not hitherto been elsewhere figured
or described, (Pl. XXX.)

The plane of the occipital foramen forms with that
of the base of the skull an angle of 140°, the plane of the posterior
surface of the skull forms with the basal plane an angle of 68°. The
occipital condyles are therefore terminal, or form the most posterior
parts of the cranium. The extent of their convex curvature in the
antero-posterior direction, which equals that of a semicircle,
indicates that the Megatherium possessed considerable freedom and
extent of motion of the head. The condyles are not extended in the
lateral direction so far as in the Toxodon; their axis is more oblique
than in the Glossotherium, and their internal surface is more parallel
with the axis of the skull, the foramen magnum not presenting that
infundibuliform expansion which is so characteristic of the
Glossotherium. The occipital condyles resemble most in form and
position those of the Scelidotherium; but in the angle of the occipital
plane the Megatherium is intermediate between the Scelidothere and
Glossothere. The ex-occipitals terminate laterally and inferiorly, each
in a short, but strong obtuse process. The posterior plane of the skull
is traversed by a strong arched intermuscular crest, which

forms the upper boundary of a pretty deep fossa,
which is divided by a median vertical ridge, extending downwards to
within an inch of the upper margin of the foramen magnum. A second
strong obtuse transversely arched ridge curves over the first, and
forms the upper boundary of the posterior or occipital region of the
skull: the interspace between the two transverse ridges is very
irregular, and indicates the firm implantation of powerful nuchal
muscles or ligaments, (Pl. XXX. fig. 1.)

In the configuration and angle of the occipital
plane the Megatherium indicates the same general correspondence with
the Edentate type, which has been pointed out in the descriptions of
the crania of the Glossothere and Scelidothere: and the resemblance to
the Scelidothere is not less striking in the small proportional size of
the cranium in this quadruped, which surpasses the rest of its class in
so great a degree in the colossal proportions of its hinder parts.

Having detected in the base of the skull of the
Scelidothere an articular semicircular pit for the head of the
styloglossal bone, similar to, but relatively smaller than, that
remarkable one in the skull of the Glossothere, it became a matter of
interest to determine whether this structure, which does not exist in
any of the existing Edentals, should likewise be present in the
gigantic type of the Megatherioid family. The result of a careful
removal of the matrix from the basal region of one of the cranial
fragments of the Megatherium was the detection of this articular
cavity, in each temporal bone in the same relative position as in the
Glossothere and Scelidothere. The styloid articular cavity is
relatively smaller, and shallower, than in the Glossothere, its
proportions being much the same as those of the Scelidothere. The
cranial or posterior extremity of the stylo-hyoid bone in the
Scelidotherium is bent upwards at an obtuse angle (Pl. XXI.), and
terminates in an articular ball which rotates in this cavity. The size
of this bone, and its mode of articulation, indicates great power and
muscularity of tongue in the Megatherioids, and calls to mind the
importance of that organ in the Giraffe, which subsists on the same
kind of food as that which I have supposed to have supported the
Megatherioids, although the general organization of these animals and
the mode in which the foliage was brought within reach of the tongue
are as opposite as can well be imagined.

The anterior condyloid foramen presents scarcely one
half the absolute size of that of the Glossothere, whence we may infer
a correspondingly inferior development of the tongue in the Megathere.
The fractured parietes of the cranial cavity of the Megatherium every
where exhibit evidences of the great extent of the air-cells or sinuses
continued from the nasal cavity: on the basilar aspect of the cranium
they extend as far back as the jugular foramina: the whole of the
basi-sphenoid being thus excavated, and permeable

to air, derived from the sphenoid sinuses, (Pl. XXX.
fig. 2.) The vertical diameter of the cranial cavity is four inches,
eight lines; its transverse diameter, which is greatest in the
posterior third part of the cavity, corresponding with the posterior
part of the cerebrum is six inches: from the indications afforded by
the remains of the cranial cavity in Mr. Darwin's specimens, I conclude
that the brain of the Megatherium was more depressed, and upon the
whole, smaller by nearly one-half than that of the Elephant; but with
the cerebellum relatively larger, and situated more posteriorly with
relation to the cerebral hemispheres: whence it may be concluded that
the Megatherium was a creature of less intelligence, and with the
command of fewer resources, or a less varied instinct than the Elephant.

It has been usual to characterize the Megatherium,
in conformity with the concurrent descriptions of Bru, Cuvier, and
D'Alton, by the dental formula of molares 4/4 4/4, i. e. by
the
presence of four grinding teeth on each side of the upper, as of the
lower jaw. It was the agreement of the excellent authorities above
cited in this statement, which induced Mr. Clift and myself to regard a
single detached tooth, which formed part of the valuable collection of
remains of the Megatherium deposited in the Hunterian Museum by Sir
Woodbine Parish, as being, from its comparatively small size, the tooth
of either a younger individual or of a smaller species of Megatherium.
Upon clearing away the matrix from the palatal and alveolar surface of
one of the cranial fragments of the Megatherium in Mr. Darwin's
collection, I was gratified by the detection of the crown of a fifth
molar, corresponding in size and form with the detached tooth, above
alluded to: its small size, and its position have doubtless occasioned
its being over-looked in the cranium of the great skeleton at Madrid.

The anterior molar of the upper jaw presents a
nearly semicircular transverse section, with the angles rounded off;
the three succeeding teeth are four-sided, with the transverse somewhat
exceeding the antero-posterior diameter: they are rather longer and
larger than the first: the last molar is likewise four-sided, but
presents a sudden diminution of diameter, and is relatively broader.
The following are the respective dimensions of the upper maxillary
teeth.

First Molar.

Second Molar.

Third Molar.

Fourth Molar.

Fifth Molar.

In.

Lines.

In.

Lines.

In.

Lines.

In.

Lines.

In.

Lines.

Length . . . . .

8

6

9

4

9

4

8

7

5

2

Transverse diameter . . .

1

9

2

4

2

3

2

0

1

4

Antero-posterior diameter . .

1

5

2

0

2

0

1

11

0

10

Besides the differences in size, the upper molars
vary as to their curvature: this difference is exhibited in the
vertical section of these teeth figured in Pl. XXXI. The convexity of
the curve of the first, second and third molars is directed

forwards; the fourth is straight, its anterior
surface only describing a slight convexity in the vertical direction;
the fifth tooth is curved, but in a contrary direction to the others;
and the bases of the five molars thus present a general convergence
towards a point a little way behind the middle of the series.

The next peculiarity to be noticed in these
remarkable teeth is the great length of the pulp-cavity (d),
the
apex of which is parallel with the alveolar margin of the jaw: a
transverse fissure is continued from this apex to the middle concavity
of the working surface of the tooth, which is thus divided into two
parts. Each of these parts consists of three distinct substances,—a
central part analogous to the body or bone of the tooth or 'dentine,' a
peripheral and nearly equally thick layer of cæmentum, and an
intermediate thinner stratum of a denser substance, which is described
in Mr. Clift's memoir on the Megatherium as 'enamel,' and to which
substance in the compound teeth of the Elephant, it is analogous both
in its relative situation, and relative density to the other
constituents.

Microscopic examinations of thin and transparent
slices of the tooth of the Megatherium prove, however, that the dense
layer separating the internal substance from the cæmentum is not
enamel, but presents the same structure as the hard 'dentine' or ivory
of the generality of Mammalian teeth; and corresponds with the thin
cylinder of hard 'dentine' in the tooth of the Sloth. No species of the
Order Bruta has true enamel entering into the composition of
its teeth; but the modifications of structure which the teeth present
in the different genera of this order are considerable, and their
complexity is not less than that of the enamelled teeth of the
Herbivorous Pachyderms and Ruminantia, in consequence of the
introduction of a dental substance into their composition corresponding
in structure with that of the teeth of the Myliobates, Psammodus,
and
other cartilaginous fishes.

The microscopic investigation of the structure of
the teeth of the Megatherium was undertaken chiefly with the view of
comparing this structure with that of the teeth of the Sloth and
Armadillo, and of thus obtaining an insight into the food, and an
additional test of the real nature of the disputed affinities of the
Megatherium. The central part of the tooth (c. Pl. XXXI.)
consists of a
coarse ivory, like the corresponding part of the tooth of the Sloth. It
is traversed throughout by medullary canals 1/1500th of an
inch
in diameter, which are continued from the pulp-cavity, and proceed, at
an angle of 50°, to the plane of the dense ivory, parallel to each
other, with a slightly undulating course, having regular interspaces,
equal to one and a half diameters of their own arcæ, and generally
anastomosing in pairs by a loop of which the convexity is turned
towards the origin of the tubes of the fine dentine, as if each pair so
joined consisted of a continuous reflected canal, (c. fig. 1,
Pl.
XXXII.) The loops are gene-

rally formed close to the fine dentine. In a few
situations I have observed one of the medullary canals continued across
the fine dentine, and anastomosing with the corresponding canals of the
cæmentum. The interspaces of the medullary canals of the coarse
dentine are principally occupied by calcigerous tubes which have an
irregular course, anastomose reticularly, and terminate in very fine
cells. The more regular and parallel calcigerous tubes, which
constitute the thin layer of hard dentine, are given off from the
convexity of the terminal loops of the medullary canals. The course of
these tubes (b. fig. 1, Pl. XXXII.) is rather more
transversely to the axis of the tooth than the medullary canals from
which they are continued. They run parallel to each other, but with
minute undulations throughout their course, in which they are separated
by interspaces equal to one and a half their own diameter. As they
approach the cæmentum they divide and sub-divide, and grow more wavy
and irregular: their terminal branches take on a bent direction, and
form anastomoses, dilate into small cells, and many are seen to become
continuous with the radiating fibres or tubes of the cells or
corpuscles of the contiguous cæmentum. This substance enters largely
into the constitution of the compound tooth of the Megatherium: it is
characterized, like the cæmentum of the Elephant's grinder, by the
presence of numerous radiated cells, or purkingian corpuscles,
scattered throughout its substance, but may be distinguished by wide
medullary canals which traverse it in a direction parallel with each
other, and forming a slight angle with the transverse axis of the
tooth. These canals are wider than those of the central coarse dentine,
their diameter being 1/1200th of an inch; they are separated by
interspaces
equal to from four to six of their own diameters, divide a few times
dichotomously in their course, and finally anastomose in loops, the
convexity of which is directed towards, and in most cases is in close
contiguity with, the layer of dense dentine.

Fine calcigerous tubes are every where given off at
right angles from the medullary canals of the cæmentum, which form a
rich reticulation in their interspaces, and a direct continuation
between the loops of the medullary canals and the calcigerous tubes of
the dense dentine. The cæmentum differs from the coarse dentine in the
larger size and wider interspaces of its medullary canals, and by the
presence of the bone-corpuscles in their interspaces; but they are
brought into organic communication with each other, not only by means
of the tubes of the dense dentine, but by occasional continuity of the
medullary canals across that substance. The tooth of the Megatherium
thus offers an unequivocal example of a course of nutriment from the
dentine to the cæmentum, and reciprocally. Retzius observes with
respect to the human tooth, that "the fine tubes of the cæmentum enter
into immediate communications with the cells and tubes of the dentine
(zahnknochen), so that this part can obtain from without the requi-

site humours after the central pulp has almost
ceased to exist." In the Megatherium, however, those anastomoses have
not to perform a vicarious office, since the pulp maintains its full
size and functional activity during the whole period of the animal's
existence. It relates to the higher organized condition, and greater
degree of vitality of the entire grinder in that extinct species.

The conical cavities (d. Pl. XXXI.)
attest
the size and form of the persistent pulp; the diameter of its base is
equal to the part of the crown of the tooth which is formed by the
coarse and fine dentine. From the gradual thinning off, and final
disappearance of these substances as they reach the base of the tooth,
I conclude that they were both formed at the expense of the pulp. The
fine tubes and cells must have been excavated in its peripheral layer
for the reception of the hardening salts of the dense dentine, and the
rest converted into the parallel series of medullary canals with their
respective systems of calcigerous tubes, in a manner closely analogous
to the development of the entire tooth of the Orycteropus. The coarser
dentine of the tooth of the Megatherium differs, in fact, from the
entire tooth of the Orycteropus, only in that the parallel
medullary canals and their radiating calcigerous tubes are not
separated from the contiguous canals by a distinct layer of cæmentum,
and that the medullary canals anastomose at their peripheral
extremities. The wide spaces, (e. Pl. XXXI.) indicate the
thickness of the dental capsule by the ossification of which the
exterior stratum of cement was formed. It was not until I knew the true
structure of the tooth of the Megatherium, that I could comprehend the
mode of its formation. The parallel layers of enamel in the Elephant's
grinder are formed, as is well known, by membranous plates passing from
the coronal end of the closed capsule towards the base of the tooth;
but a certain extent of enamel can only thus be formed, and when the
crown of the grinder has once protruded, and come into use, the enamel
cannot be added to. The modification of the structure of the tooth of
the Megatherium readily permits the uninterrupted and continuous
formation of the dense substance which is analogous to the enamel of
the Elephant's grinder.

With respect to the question of the respective
affinities of the Megatherium to the Bradypodoid or Dasypodoid
families, the result of this examination of the teeth speaks strongly
for its closer relationship with the former group: the Megalonyx,
Mylodon, and Scelidotherium, in like manner correspond in the structure
of their teeth with the Sloth, and differ from the Armadillo.

If from a similarity of dental structure we may
predicate a similarity of food, it may reasonably be conjectured that
the leaves and soft succulent sprouts of trees may have been the staple
diet of the Megatherioid quadrupeds, as of the existing Sloths. Their
enormous claws, I conclude, from the fossorial character of the
powerful mechanism by which they were worked, to have been employed,

not, as in the Sloths, to carry the animal to the
food, but to bring the food within the reach of the animal, by
uprooting the trees on which it grew.

In the remains of the Megatherium we have evidence
of the frame-work of a quadruped equal to the task of undermining and
hawling down the largest members of a tropical forest. In the latter
operation it is obvious that the immediate application of the anterior
extremities to the trunk of the tree would demand a corresponding
fulcrum, to be effectual, and it is the necessity for an adequate basis
of support and resistance to such an application of the
fore-extremities which gives the explanation to the anomalous
development of the pelvis, tail, and hinder extremities in the
Megatherioid quadrupeds. No wonder, therefore, that their type of
structure is so peculiar; for where shall we now find quadrupeds equal,
like them, to the habitual task of uprooting trees for food ?

DESCRIPTION OF FRAGMENTS OF BONES, AND OF OSSEOUS
TESSELATED DERMAL
COVERING OF LARGE EDENTATA.

It is now determined that there once existed in
South America, besides the Megatherium, the Megalonyx, and the allied
genera described in the preceding pages of the present work, gigantic
species of the order Bruta belonging to the Armadillo
family,
and defended, like the small existing representatives of that family,
by a tesselated bony dermal covering. The largest known species of
these extinct Dasypodidœ is the Glyptodon clavipes,
of
which the armour and parts of the skeleton have been described by MM.
Weiss and D'Alton in the Berlin Transactions for 1827 and 1834: and the
generic and specific characters and name, with an account of the dental
system, and bones of the extremities, were recorded in the Geological
Proceedings for March 1839. It would seem that parts of the same, or a
nearly allied gigantic species were described in the same year by M.
Lund; under the name of Hoplophorus. Of the valuable and
interesting discoveries of this able Naturalist I regret that I was not
aware until the appearance of a notice of them in the Comptes Rendus
for April, 1839.* Amongst the fragments of bony tesselated armour in
Mr. Darwin's collection are a few pieces which were found by him,
associated with remains of Toxodon and Glossotherium near the Rio Negro
in Banda Oriental.† These fragments, if we may judge from their
thickness, must have belonged to an animal at least as

* An excellent translation of the description of the
Brazilian fossils found by M. Lund, is published in the Annals of
Natural History, July and August, 1839.

† At the distance of a few leagues from the
locality here mentioned, other fragments were found by Mr. Darwin; also
near Santa Fé, in Entre Rios; also on the shores of the Laguna, near
the Guardia del Monte, South of Buenos Ayres; also, according to the
Jesuit Falkner, on the banks of the Tercero.

large as the Glyptodon clavipes; but the
pattern differs in the greater equality of size of the component
tesseræ. The thickness of the largest fragment is one inch and a half,
the tesseræ vary in diameter from one inch to half an inch, and are
separated by grooves about two lines in depth, and two in diameter. The
pattern formed by the anastomosis of these grooves is an irregular
net-work; the contour of the tesseræ is either unevenly subcircular,
hexagonal, pentagonal, or even four-sided; with the sides more or less
unequal. In those portions of this armour, where one of the tesseræ
exceeds the contiguous ones in size, the imagination may readily
conceive it to be the centre of a rosette, around which the smaller
ones arrange themselves, but there is no regular system of rosettes, as
in the portions of the dermal armour of the Glyptodon figured by Weiss,
and those brought to England by Sir Woodbine Parish, in which the
central piece is double the size of the marginal ones.

The portions of the tesselated bony dermal covering
of a Dasypodoid quadruped, figured in Pl. XXXII. figs. 5 and 4, of the
natural size, were discovered folded round the middle and ungueal
phalanges, figs. 2 and 3, at Punta Alta, in Bahia Blanca, in an earthy
bed interstratified with the conglomerate containing the remains of the
fossil Edentals.

In one of these fragments, measuring six inches long
by five broad, the tesseræ are arranged in rosettes, and so closely
correspond in size and pattern with the bony armour described by M.
Lund, as characterizing his species, Hoplophorus euphractus,
that
I feel no hesitation in referring them to that animal. One of the
pattern rosettes is figured at fig. 4, together with the thickness of
the armour at this part, and the coarse tubulo-cellular structure of
the bone. Another portion of dermal armour from the same locality,
gives the pattern shown in fig. 5, formed by square or pentagonal
tesseræ, arranged in transverse rows; it is certain that this portion
of armour belonged to the same animal as the preceding piece; and
probably that it constituted part of the transverse dorsal bands of the Hoplophorus.

The middle and ungueal phalanx, as well as the
portions of armour, are given of the natural size, in Pl. XXXII. The
upper and outer surface of the phalanx, is shown in fig. 2. It is
smooth and flat; joins the inner surface by a sharp edge, which runs
along the upper and inner side of the bone; and passes by a gradual
convexity to the under surface; the ridge corresponding with the base
of the claw, is feebly developed at the under and lateral parts of the
base of the claw. Below the double trochlear joint for the middle
phalanx, there are two articular surfaces for two large sesamoid bones.

The middle phalanx corresponds in its small
antero-posterior diameter and wedge-shape, with that of the great
Glyptodon: but the terminal phalanx is longer and deeper, in proportion
to its breadth.

Among the collection of fossils from Punta Alta, in
Bahia Blanca, there is an interesting fragment of the head of a
gigantic animal of the Edentate order, including the glenoid cavity,
and part of the zygomatic process of the left side. The articular
surface for the lower jaw, exhibits, in its flatness, extent, and the
absence of a posterior ridge, the well-marked characteristics of this
part of the Edental structure. It measures two inches four lines in the
transverse, and two inches two lines in the antero-posterior diameter.
The commencement of the zygomatic process presents a vertical diameter
of two inches, and a transverse diameter of eight lines at the thickest
part. It is slightly concave at its lower border, and convex above. The
small portion of the cranial parietes, which is preserved, exhibits the
cellular structure consequent upon the great extension and development
of the nasal air-sinuses: this condition of the cranial parietes, has
already been noticed in the description of the more perfect skulls of
the large extinct Edentata.

————————————

NOTICE OF FRAGMENTS OF MOLAR TEETH OF AMASTODON.

Of the remains of this gigantic extinct Pachyderm,
observed by Mr. Darwin at Santa Fé, in Entre Rios, and on the banks of
the Tercero, the fragments of the teeth and portions of the skeleton
which reached England, are not sufficient to lead to a determination of
the species; but sufficiently prove it to have been nearly allied, if
not identical, with the Mastodon angustidens of Cuvier, and
unquestionably distinct from the Mastodon giganteum of the
United States.

————————————

NOTICE OF THE REMAINS OF A SPECIES OFEQUUS,

Found associated with the extinct Edentals and Toxodon at Punta
Alta,
in Bahia Blanca, and with the Mastodon and Toxodon at Santa Fé, in
Entre Rios.

The first of these remains is a superior molar tooth
of the right side; it was embedded in the quartz shingle, formed of
pebbles strongly cemented together with calcareous matter, which
adhered as closely to the tooth in question, as the corresponding
matrix did to the associated fossil remains. The tooth was as
completely fossilized as the remains of the Mylodon, Megatherium, and
Scelidothere; and was so far decomposed, that in the attempt to detach
the adherent matrix, it

became partially resolved into its component curved
lamellæ. Every point of comparison that could be established proved it
to differ from the tooth of the common Equus Caballus only
in
a slight inferiority of size.

The second evidence of the co-existence of the horse
with the extinct Mammals of the tertiary epoch of South America reposes
on a more perfect tooth, likewise of the upper jaw, from the red
argillaceous earth of the Pampas at Bajada de Santa Fé, in the Province
of Entre Rios.*

This tooth is figured at Pl. XXXII. fig. 13 and 14,
from which the anatomist can judge of its close correspondence with a
middle molar of the left side of the upper jaw.

This tooth agreed so closely in colour and condition
with the remains of the Mastodon and Toxodon, from the same locality,
that I have no doubt respecting the contemporaneous existence of the
individual horse, of which it once formed part.

This evidence of the former existence of a genus,
which, as regards South America, had become extinct, and has a second
time been introduced into that Continent, is not one of the least
interesting fruits of Mr. Darwin's palæontological discoveries.

DESCRIPTION OF REMAINS OF RODENTIA, INCLUDING THE
JAWS AND TEETH OF
AN EXTINCT SPECIES OFCTENOMYS.

The fragment of the upper jaw, figured in Pl. XXXII.
fig. 6, exhibits the first and second molar in situ, and the
socket of the third and fourth molar, of a Rodent, which by the form
and number of the upper maxillary teeth is referable to the genus
Ctenomys. The molars are a little larger, the longitudinal groove on
their external surface is somewhat deeper, and the last molar is
relatively wider than in the existing subterraneous species,—the
Tucutucu (Ctenomys Brasiliensis, Bl.), of whose habits so
interesting an account is given in the description of the Mammalia of
the present Collection (No. IV. p. 79). The form of the grinding
surface of the first and second upper molar is shown below the fig. 6,
and three views of the second grinder are given at figs. 7, 8, and 9.
The fragment of the lower jaw of the same fossil Rodent is figured at
fig. 10 and 11. The long anterior incisor is relatively narrower than
in the Ctenomys Brasiliensis. I have not had the means of
comparing this fossil with the Ctenomys Magellanicus; but
since it is probable that the Ct. Magellanicus may not be
specifically different from the Ct.

* Mr. Darwin has more particularly described the
circumstances of
the embedment of this tooth in his Journal of Researches, p. 149,
during the Voyage of the Beagle.

Brasiliensis, it may be concluded that the present fossil
is
equally distinct from both.

The portion of the right hind-foot of the Rodent
figured at fig. 12, includes the calcaneum, astragalus, cuboides,
external and middle cuneiform bones, and the metatarsals and proximal
phalanges of the toes corresponding with the three middle toes of
five-toed quadrupeds. The metatarsals are chiefly remarkable for the
well-developed double-trochlear articular surface, and intermediate
ridge. These remains, as well as the jaws and teeth of the Ctenomys,
were discovered at Monte Hermoso in Bahia Blanca.

In the same reddish earthy stratum of that locality,
Mr. Darwin discovered the decomposed molar of a Rodent, equalling in
size, and closely resembling in the disposition of its oblique
component laminæ, the hinder molar of the Capybara (Hydrochærus.)
The
fossil differs, however, in the greater relative breadth of the
component laminæ.

I have, lastly, to notice the head of a femur, and
some fragments of pelvic bones from the same formation which bear the
same proportion to the tooth above alluded to, as subsists between the
teeth and bones of the Capybara, and which are sufficient to prove that
there once has existed in South America a species of the family Caviidæ, as large as the present Capybara, but now apparently
extinct.

This fact, together with the greater part of those
which have been recorded in the foregoing pages of the present work,
establishes the correspondence, in regard to the characteristic type,
which exists between the present and extinct animals of the South
American Continent: we have abundant evidence likewise of the greater
number of generic and specific modifications of these fundamental types
which the animals of a former epoch exhibited, and also of the vastly
superior size which some of the species attained.

At the same time it has been shewn that some of the
present laws of the geographical distribution of animals would not have
been applicable to South America, at the period when the Megatherioids,
Toxodon, and Macrauchenia existed: since the Horse, and according to M.
Lund, the Antelope and the Hyæna, were then associated with those more
strictly South American forms. The Horse, which, as regards the
American continent, had once become extinct, has again been introduced,
and now ranges in countless troops over the pampas and savannahs of the
new world. If the small Opossums of South America had been in like
manner imported into Europe, and were now established like the
Squirrels and Dormice in the forests of France, an analogous case would
exist to that of the Horse in South America, as the fossil Didelphys of
Montmartre proves.

With respect to the geological contemporaneity of
the fossils collected by him, Mr. Darwin subjoins the following
observations:—

"The remains of the following animals were embedded
together at Punta Alta in Bahia Blanca:—The Megatherium Cuvierii, Megalonyx Jeffersonii, Mylodon Darwinii, Scelidotherium
leptocephalum, Toxodon Platensis (?) a Horse and a small Dasypodoid
quadruped,
mentioned p. 107; at St. Fé in Entre Rios, a Horse, a Mastodon, Toxodon
Platensis, and some large animal with a tesselated osseous dermal
covering; on the banks of the Tercero the Mastodon, Toxodon, and,
according to the Jesuit Falkner, some animal with the same kind of
covering; near the Rio Negro in Banda Oriental, the Toxodon
Platensis, Glossotherium, and some animal with the same kind of
covering. To these two latter animals the Glyptodon clavipes,
described
by Mr. Owen in the Geological Transactions, may, from the locality
where it was discovered, and from the similarity of the deposit which
covers the greater part of Banda Oriental, almost certainly be added,
as having been contemporaneous. From nearly the same reasons, it is
probable that the Rodents found at Monte Hermoso in Bahia Blanca,
co-existed with the several gigantic mammifers from Punta Alta. I have,
also, shown in the Introduction, that the Macrauchenia
Patachonica, must have been coeval, or nearly so, with the last mentioned
animals. Although we have no evidence of the geological age of the
deposits in some of the localities just specified, yet from the
presence of the same fossil mammifers in others, of the age of which we
have fair means of judging, (in relation to the usual standard of
comparison, of the amount of change in the specific forms of the
invertebrate inhabitants of the sea,) we may safely infer that most of the animals described in this volume, and likewise the
Glyptodon, were strictly contemporaneous, and that all lived
at about the same very recent period in the earth's history. Moreover,
as some of the fossil animals, discovered in such extraordinary numbers
by M. Lund in the caves of Brazil, are identical or closely related
with some of those, which lately lived together in La Plata and
Patagonia, a certain degree of light is thus thrown on the antiquity of
the ancient Fauna of Brazil, which otherwise would have been left
involved in complete darkness."

BY GEORGE R. WATERHOUSE, Esq. Curator of The Zoological Society of London, &c.

This Division of the Work is now complete, Price £1 18s.

NOS. III. VI. IX. AND XI.

BIRDS.

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